This application relates to and incorporates herein by reference Japanese Patent Applications No. 11-222031 filed Aug. 5, 1999, No. 11-223065 filed Aug. 5, 1999 and No. 11-230430 filed Aug. 17, 1999.
1. Field of the Invention
The present invention relates to a variable valve timing control apparatus for internal combustion engines.
2. Related Art
The use of an internal combustion engine using a variable valve timing control apparatus in a vehicle is now increasing to enhance its output, to save fuel consumption and to reduce emission of exhaust gas. A vane type variable valve timing control apparatus comprises, as shown in FIG. 36, a housing 1 which rotates or turns in synchronism with a crankshaft of the engine, a rotor 2 connected to an intake or exhaust valve camshaft and disposed concentrically with the housing 1, and fluid chambers 3 formed in the housing 1 and parted as advancing chambers 5 and retarding chambers 6 by vanes 4 provided on the rotor 2.
The valve timing is variably controlled by changing rotational angular phase of the camshaft or xe2x80x9cphase of camshaftxe2x80x9d with respect to the crankshaft by relatively turning the housing and the rotor 2 (vanes 4) by controlling hydraulic pressure of the advancing chamber 5 and the retarding chamber 6 by a hydraulic pressure control valve.
JP-A-9-324613 proposes a method of enlarging a controllable range of the valve timing (phase of camshaft) by setting lock phase in stopping the engine almost at the intermediate position of the controllable range of the camshaft phase.
However, in this arrangement, the hydraulic pressure of the both advancing and retarding chambers 5 and 6 operates always in the unlocking direction. As a result, a lock pin 7 is unlocked sometimes when the hydraulic pressure of either one of the advancing chamber 5 and the retarding chamber 6 first becomes high, even though the hydraulic pressure of the other one is low, due to the increase of the hydraulic pressure caused by the increase of engine speed (number of revolutions of oil pump) in cranking the engine.
The camshaft phase changes suddenly at the moment of unlock and the valve timing cannot be controlled to the target value because the hydraulic pressure of the other one is low even when the lock is released in such state. As a result, because the engine is cranked at the inadequate valve timing, the startability is aggravated, the engine start time is prolonged or the engine operating state after the start becomes unstable until when the hydraulic pressure increases. Still more, the vane 4 hits the housing 1, thus causing noise, because the position of the vane 4 is not fixed until when the hydraulic pressure increases.
The camshaft phase also changes suddenly to the side where the hydraulic pressure is high at the moment of unlock, thus causing the actual valve timing (phase of camshaft) to deviate largely from the target value. Therefore, an abnormality checking system for determining whether or not the variable valve timing control apparatus is abnormal erroneously determines sometimes that the transient behavior of the camshaft phase in unlocking (releasing the lock) is abnormal.
When the engine is stopped during cold ambient conditions before the oil temperature rises, oil is hardly pulled out of a lock hole. The lock pin 7 is hardly fitted into the lock hole when the engine is stopped because the viscosity of the oil within the hydraulic circuit is large and the fluidity of the oil is bad. The lock pin 7 is hardly fitted into the lock hole, when the motion of the camshaft phase is slow due to a fault or the like. When the lock pin 7 is unlocked while operating the engine in these cases, the lock pin 7 is hardly fitted into the lock hole and the camshaft phase cannot be locked at the intermediate lock phase in stopping the engine afterward.
When the camshaft phase cannot be locked at the intermediate lock phase in stopping the engine, the valve timing (phase of camshaft) cannot be controlled to the target value (around the intermediate lock phase) until when the engine speed increases and the hydraulic pressure increases. As a result, because the engine is cranked with valve timing deviated away from the target value, startability is bad and it takes time to crank the engine. Still more, because the position of the vane 4 is not fixed until the hydraulic pressure increases, when the engine is cranked without locking the camshaft phase, the vane 4 hits the housing 1, thus causing noise.
Accordingly, a primary object of the invention is to provide a variable valve timing control apparatus for internal combustion engines which can reliably prevent the camshaft phase from being unlocked in cranking the engine.
A secondary object of the invention is to improve the reliability of a system for checking abnormality by preventing it from erroneously determining the transient behavior of the camshaft phase right after the unlock as being abnormal in advance.
A tertiary object of the invention is to avoid the problems of the bad startability, noise and others caused by the failure of the lock by preventing the engine from cranking in the state in which the camshaft phase is not locked.
In order to achieve the primary object of the invention, a fluid pressure control is controlled so that no fluid pressure which otherwise causes unlock is applied to a lock releasing chamber of a locking part in cranking an engine. Thereby, it becomes possible to reliably prevent the camshaft phase from being unlocked in cranking the engine and to improve startability, controllability of the engine and noise which are otherwise caused by the careless unlock.
In order to achieve the secondary object of the invention, abnormality check condition is relaxed or lowered in unlocking the lock part. Thereby, it becomes possible to prevent the transient behavior of the camshaft phase right after the unlock from being erroneously determined as being abnormal in advance. Still more, because the process for checking abnormality may be continued by the relaxed abnormality check condition even when the lock is unlocked, the abnormality may be detected quickly if the abnormality has occurred. Thus, it is possible to achieve both the functions of detecting abnormality quickly and of preventing erroneous detection.
In this case, the abnormality check condition may be returned to normal values after an elapse of a certain period after detecting that the lock part is unlocked. Thereby, the abnormality check condition may be returned after the control of the camshaft phase is stabilized after the unlock.
The period until when the control of the camshaft phase is stabilized after the unlock changes depending on the viscosity of oil (fluidity of oil) within a hydraulic circuit at that time and the viscosity of oil changes depending on oil temperature. Because the oil temperature correlates with coolant temperature and engine temperature, the coolant temperature or the engine temperature may be used as substitute information of the oil temperature.
In order to achieve the tertiary object of the invention, unlock of the lock part may be inhibited until when the camshaft phase becomes movable after cranking the engine. Thereby, the internal combustion engine may be stopped in the state in which the camshaft phase is locked by the lock part by stopping the internal combustion engine before the camshaft phase becomes movable after cranking the engine. Therefore, it becomes possible to crank the engine next time in the state in which the camshaft phase is locked firmly by the lock part and to avoid aggravation of startability and noise which are otherwise caused by the failure of the lock.
In this case, it is possible to check whether or not the camshaft phase is movable based on information on temperature such as oil temperature, coolant temperature and engine temperature by taking into account that the motion of the camshaft phase changes depending on the viscosity (oil temperature) of oil within the hydraulic circuit.